1. Field of the Invention
This invention relates to a base station in a Code Division Multiple Access (CDMA) mobile communication system, and more particularly, to a demodulation apparatus of a base station in CDMA mobile communication system.
2. Background of the Related Art
Quadrature Phase Shift Keying (QPSK) demodulation in a mobile communication system may be accomplished by converting a high frequency signal received via an antenna into an intermediate frequency signal by using a mixer, and dividing the intermediate signal into an In-Phase channel (channel) and a Quadra-Phase channel (Q channel) before demodulating to a base-band frequency signal with a demodulator. The analog base-band frequency signal thus demodulated into I and Q channels using the QPSK demodulation is then adjusted to a specific amplitude using a Low-Pass filter and an amplifier. Next, the signal is converted into a base-band digital signal, which is a CDMA signal, using an analog-to-digital converter. Finally, the CDMA signal is transferred to a CDMA modem for demodulation.
FIG. 1 shows a block diagram illustrating a related art base station receiver of a mobile communication system. Referring to FIG. 1, a signal transmitted from a mobile station is received through the base station antenna and relayed to a low noise amplifier 103 via the channel-pass filter 102. The low noise amplifier 103 then amplifies and outputs the signal, and passes it through the band-pass filter 104 and the high frequency amplifier 105. Next, the signal is converted into an intermediate frequency CDMA signal in the high frequency mixer 106.
The intermediate frequency CDMA signal then passes through a Surface Acoustic Wave (SAW) filter 107 and the intermediate frequency amplifier 108 to remove unnecessary signal components. The resulting signal is transmitted to the QPSK demodulator 109 where it is demodulated. The QPSK demodulator 109 outputs an I channel signal and a Q channel signal.
The high frequencies of the analog I channel and Q channel signals as demodulated by the QPSK demodulator 109 are removed by the low-pass filter 110. The filtered signals are then amplified by the base-band amplifier 111 to a specific amplitude compatible with the analog-to-digital conversion, and subsequently transferred to the Analog-to-Digital converter 112.
The Analog-to-Digital converter 112 converts the transmitted signal into a base-band digital signal. The base-band CDMA signal (i.e., the I channel and the Q channel converted to digital signals) is relayed to a CDMA modem 113 for demodulation. Thereafter, the demodulated CDMA signal is transferred to the central office 114, for example a switching station, as a Pulse Code Modulation (PCM) signal.
The QPSK demodulation method of the related art system, however, has various problems. For example, the signal is divided into the analog base-band frequency signal with an I channel and a Q channel by the QPSK demodulator 109, and the divided signal is converted to the digital signal using the Analog-to-Digital converter 112. As a result, numerous circuit devices are needed, making the circuit diagram is more complex, more expensive, and less efficient.